Well, went down to the car in the garage this morning, started her up, and thought whilst thinking I would start the dismantling process of the back shelf, " let's just push the blind button". The blind worked straight away!!!!

What a relief, as I have got better things to do than take out the rear shelf!
It had occurred to me that, ironically for a sun blind, the high heat we have had down here over the last few days at least had something to do with "the problem". When I first went to use the blind it was in Sainsbury's supermarket car park with external temperature running at 27 degrees C, and the whole rear window area baking in direct heat, so what that temperature was, who knows!
Since then, over the last couple of days the temp has kept high, but this morning it has dropped, and the car was in a cool garage. That, no doubt, is the reason why it has come back to life. I have wiped all the mechanism in polish, and checked the metal trim that surrounds the opening I created in the shelf when I retrofitted the blind. It could be that expanded just enough to impede the blinds upward movement.
Anyway, thanks for all your input!